Reducing intake from large portions is of critical importance to preventing obesity. People consistently eat more when they are served larger portions, a phenomenon known as the portion size effect. The mechanisms of the portion size effect are not well understood, and investigating the underlying neurobiology that drives this phenomenon may inform the development of more effective obesity prevention programs. The proposed research will follow healthy weight children who vary by family risk for obesity to identify the neurobiological and appetitive traits that are implicated in overeating and weight gain during the critical pre-adolescent period. Our central hypothesis is that increased intake from large portions of energy dense foods is due in part to reduced activity in brain regions implicated in inhibitory control and decision making, combined with increased activity in reward processing pathways. To test this hypothesis, we will recruit 120, 7-8 year-old healthy weight children at two levels of obesity risk (i.e., 60 high-risk and 60 low-risk) based on parent weight status. In aim one, we will use functional magnetic resonance imaging to characterize the brain regions that are activated in response to food portion size and compare these regions between high and low-risk children. Second, we will determine the relationship between brain response to visual portion size cues and measured food intake when portions are increased in the laboratory. Third, we will determine the relationship between brain response to large portions and other validated measures of overeating, including satiety responsiveness and tendency to eat when not hungry. Fourth, we will conduct follow-up visits one year after baseline to determine the extent to which baseline brain and behavioral responses to portion size predict gains in adiposity. The primary innovations offered by this proposal are the use of both prospective and family-risk designs which may provide insight into the causal pathway linking portion size response to weight gain during the vulnerable pre-adolescent period. Second, we will use a ?brain-as-predictor? framework to study the neurobiological mechanisms of the portion size effect and its relationship to obesity. Finally, we will employ sophisticated statistical techniques that will allow us to develop more generalizable models that include both brain and behavioral data to predict increases in body fat over time. We expect these results to confirm the hypothesis that reduced function of brain inhibitory pathways and increased activity in brain reward pathways in response to portion size cues contributes to excess intake with large portions and greater weight gain over time, particularly in children who have higher risk for obesity. The proposed studies will characterize the relationship between brain response to portion size and eating behavior and will allow us to determine whether brain and behavioral responses predict body fat gain during pre-adolescence. These studies will contribute essential information to our understanding of the pathways implicated in overeating and obesity and will facilitate the characterization of ?at risk? phenotypes that can be targeted by prevention programs.